While transmission speeds are pushed into the Tbit range for modern, multi-channel telecommunication systems, a large interest is pointed towards new optical fibres with relatively large mode areas. One of the key issues is to avoid disturbance of individual channels, while these are transmitted over a fibre link. The disturbance is mainly related to non-linear effects in the fibres—effects that can be suppressed or in practice eliminated by the use of fibres with large mode areas and/or special dispersion properties.
For many applications, a large mode area can, however, not be tolerated using presently known fibres, since such fibres exhibit high macro-bending losses. This is e.g. the case for applications where fibres are used in compact modules and consequently must be coiled with relatively small bending radil—typically around 6 cm—such as for dispersion compensating fibre modules. The present invention solves the problem of macro-bending sensitive large-mode area fibres, and provides robust, single-mode, large-mode area fibres for long-distance optical transmission and for fibres with special dispersion properties such as dispersion-shifted fibres, dispersion compensating and dispersion slope compensating fibres. Typically, optical fibres for dispersion compensation have a core diameter of around 4 μm, and the present invention provides fibres with core diameter larger than 4 μm.
Recently a new type of optical fibre that is characterized by a so-called micro-structure has been demonstrated. Optical fibres of this type (which are referred to by several names—e.g. micro-structured fibres, photonic crystal fibres, holey fibres, or photonic bandgap fibres) have been described in a number of references, such as WO 99/64903, WO 99/64904, and Broeng et al. (see Pure and Applied Optics, pp.477-482, 1999) describing such fibres having claddings defining Photonic Band Gap (PBG) structures, and U.S. Pat. No. 5,802,236, Knight et al. (see J. Opt. Soc. Am. A, Vol. 15, No. 3, pp. 748-752, 1998), Monro et al. (see Optics Letters, Vol.25 (4), p.206-8, February 2000) defining fibres where the light is transmitted using modified Total Internal Reflection (TIR). This invention concerns mainly fibres that are guiding by TIR.
Micro-structured fibres are known to exhibit waveguiding properties that are unattainable using conventional fibres. One of these unique properties is that micro-structured fibres can be designed to be so-called endlessly single mode (see Birks et al. Optics Letters, July 1, 22(13), pp. 961-963, 1997). Such fibres have a very important aspect, namely that they, in principle, can be designed with a very large mode area at any desired wavelength while remaining single mode (see Birks et al. Electronics Letters, June 25, 34(13), pp. 1347-1348, 1998, and WO 99/00685). Although endlessly single mode fibres known from the prior art may, in theory, have infinitely large mode areas, the fibres will in practice have a mode area that is limited by macro-bending losses (see Sørensen et al. Electronics Letters, Vol.37, no.5, 1st Mar. 2001 and Sørensen et al. 27th European Conference on Optical Communication, paper We.P.1, Amsterdam, 2001). As demonstrated in the literature, the prior art, endlessly single mode fibres must have a filling fraction of the micro-structured cladding that is lower than a certain critical value. Often the micro-structured cladding is formed using periodically arranged air holes in a silica background material, as for the fibres in the above-cited references. The critical air filling fraction of prior art endlessly, single mode micro-structured fibre designs has been investigated both experimentally by Knight et al. (see the above-cited Knight reference) and theoretically by Broeng et al. (see Broeng et al, Optical Fiber Technology, Vol. 5, pp.305-330 1999). In theory, for fibres made from pure silica, having circular air holes placed in a close-packed arrangement—or triangular arrangement—a critical filling fraction of 18% has been found. In practice, for prior art fibres a lower filling fraction—of less than 5%—has been found. Hence, fibres known from the prior art are not able to be endlessly single-mode (and, therefore, to have a very large mode area) unless the filling fraction is below at least 18%. For close-packed arrangement of air holes in silica, the critical air filling fraction of 18% corresponds to a hole diameter, d, of around 0.45 times the center-to-centre spacing, Λ, of to nearest air holes.
The present inventors have realized that a more appropriate parameter with respect to the cut-off properties of micro-structured fibres, is the minimum spacing, w, between boundaries of two nearest holes. By geometrical considerations, w may be deduced to be equal to Λ−d. Hence, the above-mentioned critical air filing fraction of 18%—or critical hole diameter of 0.45Λ—corresponds to a critical (minimum) w value of 0.55Λ.
The critical filling fraction is an important parameter for practical applications, as it is one of the key parameters that determine the robustness of a micro-structured fibre in terms of low macro-bending losses. A large filling fraction is a general requirement in order to eliminate macro-bending losses.
It is a disadvantage of single-mode, micro-structured fibres with core diameters above 4 μm, which are known in the prior art, that they have filling fractions of 18% or smaller. The present invention discloses a number of new designs of relatively large mode area, single mode fibres with larger critical air filling fraction than known from the prior art, as well as a method for producing fibres with such designs. This is achieved be using two concentric cladding regions—an inner and an outer cladding region—where the outer cladding region has a filling fraction larger than 18% and the inner cladding region is designed to have w around 0.55Λi, where Λi is the center-to-centre spacing of two nearest features in the inner cladding. Hence, in the case of both the inner and outer cladding features being circular air holes, the inner cladding features have a diameter, di, of around 0.45Λi, whereas the outer cladding features have a diameter, do, larger than 0.50Λo (Λo being the center-to-centre spacing of two nearest features in the outer cladding). The larger filling fraction in the outer cladding provides improved macro-bending losses properties, whereas the design of the inner cladding features ensures single mode operation. Hence, the present invention provides greater flexibility when designing fibres having relatively large mode areas (larger than 4 μm in core diameter) with respect to macro-bending losses and/or dispersion properties. Fibres according to the present invention will be less sensitive to macro-bending losses than presently known single-mode fibres with similar sized mode areas—due to the special relation between the dimensions of the inner and outer cladding features. Furthermore, single-mode fibres according to the present invention may be utilized for applications, where a relatively large mode area and specifically tailored dispersion properties are of importance, such as dispersion compensating fibres and dispersion slope compensating fibres. The fibres are, however, not only of interest in optical telecommunication, but also for very high power light transmitting systems, such as e.g. laser machining and medical surgery, as well as the fibres are of interest for sensing systems.